Somatosensory evoked potential (SEP) peak amplitude has been shown to correlate with pain sensation when the level of noxious stimulation is varied. Operantly conditioned changes in SEP peak amplitude have been shown to produce significant changes in pain sensitivity. Thus, the SEP holds considerable promise for providing a non-invasive means of measuring and controlling pain-related physiological processes in man. Previous research in both these areas has been hampered by the multiple component nature of the SEP peaks. Components generated by non-pain-related neurons can mask those generated by pain-related neurons and obscure the relationship between the SEP peak and pain processes. The proposed work will investigate the relationship between the SEP and pain processes using procedures that will better separate and identify the different components underlying the SEP peaks. First, an indifferent reference electrode site will be determined to minimize contamination by components recorded from the reference site. Second, a quantitative scalp topographic analysis, the first of its kind in pain-related SEP studies, will determine the temporal and spatial (amplitude at each scalp location and pattern) features of the SEP that best correlate with perceived magnitude ratings and changes in the level of noxious and innocuous afferent input. Third, the P180 peak of the SEP will be operantly conditioned and its effect on pain sensitivity investigated. Control groups will evaluate whether the change in pain sensitivity is due to non-specific psychological factors, e.g., expectation, attention, etc., or is specific to conditioned changes in the SEP. The SEP scalp topography will be analyzed to determine whether spatial and temporal features of the SEP best related to pain sensitivity observed during conditioning are similar to those determined in the experiments that varied afferent input.